Surface analysis for materials characterisation involves directing a beam of primary particles (photons, electrons, ions etc.) to the surface of a sample and measuring the energy or mass of the secondary particles (photons, electrons, ions etc.) emitted from the surface as a result. Control of the electric potential of the sample surface is important, to ensure accurate measurements can be taken.
In monochromated XPS analysis, a primary beam of monochromatic X-ray photons causes the emission of secondary electrons from the sample surface. For an insulating sample (such as an insulator, an electrically isolated conductive sample, a conductive sample with an insulating surface, or a sample including one or more electrically isolated regions), the emission of secondary electrons leaves the analysis region positively charged. It is known to use an electron flood gun to provide charge compensation for the electrons emitted from the sample surface. The purpose of the flood gun is to charge the surface to a negative potential, approximately equivalent to the energy of the incident electrons in the flood, so that a dynamic steady state is set up, in which flood electrons reach the sample surface at the same rate as the secondary electrons leaving the surface and any excess flood electrons are reflected from the surface.
In practice, however, a uniform surface potential is not provided. Any areas of the sample surface which are not irradiated by X-rays, but receive incident electrons, become charged to the potential of the most energetic incident electrons. These areas then act to repel flood electrons which are needed to replace the photoemitted electrons from the analysis region which is irradiated by X-rays. The net negative charge developed around the irradiated, analysis region also acts to defocus the electron beam from the flood gun. These effects are particularly severe when the sample is much larger than the analysis region.
It is known to use a beam of low-energy, positive ions to neutralise this build-up of negative charge.
Larson and Kelly, “Surface charge neutralisation of insulating samples in X-ray photoemission spectroscopy”, Journal of Vacuum Science and Technology A, 16(6), November/December 1998, pp 3483-3489, indicate that the principal cause of the negative potential in the region surrounding the analysis region is the energy spread of the flood electrons from the electron flood gun, which includes a high-energy tail. Larson and Kelly use a neutralising system in which a high current density flood gun with a narrow energy spread is used in combination with a source of low-energy, positive ions.
GB-A-2,411,763 provides a background discussion of the above problem and discloses a combined flood gun, which provides a source of electrons and a source of positively charged particles and is capable of focusing the electron flood beam and the positive particle beam independently towards the sample surface. Although the combined flood gun of GB-A-2,411,763 provides an effective instrument for neutralising the unwanted electrons at the sample surface, facilitating further the control of the potential of the sample surface would be desirable. The invention aims to address the above and other objectives by providing an improved XPS analysis system for surface analysis.